[DocsUI] Add item_root to medium Material 3 directory list layout

The existing Material 3 medium screen size layout does not put the
item_root ID on the file row, which the other layouts do. This was
causing tests in PickerPreviewAllTypeUiTest to fail.

Flag: EXEMPT bugfix
Bug: 430421551
Test: atest DocumentsUIGoogleTests.com.android.documentsui.PickerPreviewAllTypeUiTest
Change-Id: Id99092b6e6395e6298b9babdbe20c635d399d3e7